Electrical heating sheet with series of eyelets connections

ABSTRACT

Conductive silicone rubber heating elements having metal foil electrodes attached by eyelets formed by punching the foil through the conductive sheet.

United States Patent MacColl et a1.

[54] ELECTRICAL HEATING SHEET WITH SERIES OF EYELETS CONNECTIONS [72] Inventors: Donald MacColl, Ardrossan; Hugh OPray, Stevenston, both of Scotland [73] Assignee: Imperial Chemical Industries Limited, London, England [22] Filed: Feb. 16, 1971 21 App1.No.: 115,846

[30] Foreign Application Priority Data March 9, 1970 Great Britain ..11,179/7O [52] US. Cl. ..219/549, 219/528, 219/541, 219/543, 338/212, 339/220 R [51] Int. Cl. ..II05b 3/34 [58] Field of Search ..219/549, 543, 528, 545, 527, 219/529, 541; 338/208-212, 312; 174/117 M; 339/17, 220 R, 276

[56] References Cited UNITED STATES PATENTS 1,333,724 3/1920 Meunier ..338/293 X [451 Se t. 12, 1972 1,374,639 4/1921 Dicks ..338/293 2,287,915 6/1942 Taylor ..219/211 3,197,729 7/1965 Sarazen ..339/220 R X 2,759,092 8/1956 Fortin ..338/208 3,387,248 6/1968 Rees ..338/211 2,559,077 7/1951 Johnson et a1 ..219/543 3,385,959 5/1968 Ames et a1 ..219/549 1,962,041 6/1934 Spong ..174/117 M 2,421,953 6/ 1947 Mackendrick ..219/549 X FOREIGN PATENTS OR APPLICATIONS 108,498 9/1939 Australia ..219/528 501,444 2/1939 Great Britain ..219/5 28 1,079,902

8/1967 Great Britain ..338/211 Primary Examiner-Volodymyr Y. Mayewsky Attorney-Cushman, Darby & Cushman [5 7] ABSTRACT Conductive silicone rubber heating elements having metal foil electrodes attached by eyelets formed by punching the foil through the conductive sheet.

2 Claims, No Drawings ELECTRICAL HEATING SHEET WITH SERIES OF EYELETS CONNECTIONS This invention relates to heating elements and more.

particularly to electrical heating elements of the kind comprising a sheet of electrically conducting material provided with two or more electrodes for the passage of electricity therethrough.

Heating elements in which heat is provided by the passage of electricity through a sheet of conducting material are widely known. The conducting material is generally a thermoplastic or thermosetting resin or plastic or a rubber rendered electrically conducting by the incorporation therein of particles of an electrically conducting material such as carbon black. Among suitable conducting materials are the electrically conducting organopolysiloxane elastomers which are particularly useful because of their temperature resisting properties and because they have a positive temperature coefficient of resistance. These conducting materials may be simply in sheet form or may be coated on a woven, knitted or felted fabric or on a film, for example, such as paper, glasscloth, a polyester fabric or a polyester film.

Electric current isfed into the sheet of conducting material through two or more electrodes connected to a source of electricity. Among the materials used as electrodes have been metal foils such as copper, aluminum or brass foils. These foils have been fastened to the sheet of conducting material by a variety of methods including stitching by one or more rows of stitching or by oversewing, stapling, cementing or vulcanizing and inserting into pockets formed by the conducting sheet material.

According to the present invention a heating element comprises a sheet of electrically conducting material having two or more metal foil electrodes attached thereto, the electrodes being fastened to the sheet by a series of eyelets formed in the electrodes such that the portions of the eyelets on the underside of the sheet clamp the electrodes thereto and being capable of connection to an electrical power supply.

The conducting sheet used in the heating elements of our invention may be formed of supported or unsupported electrically conducting material such as conducting natural or synthetic rubber, conducting polyolefins, conducting organopolysiloxane elastomers or graphite/silica mixtures. lt is frequently preferred that the conducting material should be an electrically conducting organopolysiloxane elastomer. It is also, in general, preferred that the electrically conducting material forming the sheet be supported by a substrate, in which case one or both sides thereof may be coated or impregnated with the electrically conducting material. The substrates hitherto used for this purpose such as, for example, paper, asbestos paper, woven or nonwoven polyester or nylon fabrics, glasscloth, films of polyesters or other high melting polymers, may be used.

For many purposes it is convenient and in fact, preferred to use glasscloth.

The conducting sheet may be produced in known manner by forming an unsupported sheet or by coating or impregnating the desired substrate, for example, by knife coating, dipping or spraying, followed by any treatment necessary to stabilize the sheet such as curing or vulcanization.

The eyelets formed in the electrodes may be formed by the normal types of eyeletting tools, it being necessary only that the turned over portion of the eyelets should be of sufficient size to clamp the electrode to the conducting sheet. The eyelets may, of course, vary in size and spacing. Thus if they are made too small in diameter the stage may be reached at which the eyelets exercise insufficient clamping action and the electrodes will no longer be held to the sheet of conducting material. If on the other hand they are made too large only a limited amount of electrode remains and this results in their being insufficient electrode surface to transmit the electrical power necessary unless the total area covered by the electrode is increased.

In some cases it may be considered desirable to have a reinforcing strip which is held to the underside of the conducting sheet by the turned over portion of-the eyelets. If a reinforcing strip is used it may be of any suitable material which may or may not be electrically conducting, for example metal foil, plastic strip or other similar material. In many cases a non-woven strip of polyester is eminently suitable. It is frequently convenient and is generally preferred, if a reinforcing strip is used, that it be of the same or similar foil as is used for the electrodes.

Our invention is further illustrated by the following Examples.

EXAMPLE 1 A heating element was made up by attaching two electrodes along opposite edges of a sheet of glasscloth coated on both sides with an electrically conducting organopolysiloxane elastomer. The sheet was 36 inches by 13 inches and of resistance 320 ohms per square. The electrodes were of copper foil 0.155 mm thick and 6.35 mm wide and were fastened to the coated sheet along the longer sides by a line of eyelets of 2.0 mm diameter and 6.35 mm apart passing through the sheet and through an underlying copper foil 0.08 mm thick and 6.35 mm wide. The power dissipated by the heating element was 54 watts when connected to an volt electrical supply.

EXAMPLES 2 to 13 A number of heating elements similar to those of Example l was made up and the contact resistance between the electrode and conducting sheet measured in each case. The table below shows details of the elements. The electrodes were in all cases 6.35 mm wide and 0.155 mm thick and the backing strips where used were 6.35 mm wide and of the thickness shown in the table.

7 Woven 0.l Soft None 82 polyester Copper 8 Felted 0.125 Soft Soft 0.078 79 polyester Copper Copper 9 Felted 0.125 Soft None 96 polyester Copper l polyester 0.05 Soft Soft 0.078 46 film Copper Copper ll polyester 0.05 Soft None 70 film Copper 12 Felted 0.625 Soft Soft 0.078 193 Nylon Copper Copper l3 Felted 0.350 Soft Soft 0.078 149 polyester Copper Copper EXAMPLES l4 15 Two heating elements similar to those of Example 1 were made up the size being 7 inches X 7 inches and the resistance of the coated glasscloth sheet being 340 ohms per square. The electrodes were of soft copper 6.35 mm wide and 0.155 mm thick and there was a soft copper reinforcing strip 0.078 mm thick. The two elements differed only in the size and spacing of the eyelets. These dimensions together with the contact resistance of the electrodes and pull required to strip off the electrodes at a rate of 12 inches per minute are given in the table below:

Example Eyelets Contact Pull Resistance dia. (ohm cm) (mmblo/cm. (lb) What we claim is:

1. A heating element comprising a sheet of electrically conducting material consisting of a layer of electrically conducting organopolysiloxane elastomers supported on a layer of a glass cloth substrate and having at least two metal foil electrodes attached thereto, each of the electrodes being fastened to the sheet by a series of eyelets integrally formed in the electrodes such that the portions of the eyelets on the underside of the sheet clamp the electrodes thereto and a reinforcing strip held to the underside of the conducting sheet by the turned over portion of the eyelets, said electrodes being capable of connection to an electrical power supply.

2. A heating element according to claim 1 wherein the reinforcing strip is a metal foil similar to that used for the electrodes. 

2. A heating element according to claim 1 wherein the reinforcing strip is a metal foil similar to that used for the electrodes. 